Various techniques are known in the art for detecting the level of liquid in a container. Some techniques are based on detecting the initial contact between a sensing element and the surface of the liquid. The initial contact may result from the sensing element moving toward the liquid surface or may occur where the sensing element is fixed with respect to the container and the liquid level rises to meet the element. In either instance, a device connected to the sensing element detects a change in some parameter sensed by the probe which indicates contact between the sensing element and the liquid surface
Other techniques use a sensing element that is fixed within the container and which extends over an anticipated range of liquid levels. As the liquid level changes within the container, a device connected to the sensing element measures a parameter which changes with the liquid level to provide a continuous indication of liquid level.
With either type of liquid level detecting technique, it is known to use capacitance as the parameter that is sensed by the sensing element. Several problems exist, however, with liquid level detecting techniques based on capacitance measurement. For example, the ability to detect very small changes in capacitance, that is, sensitivity, is a common problem. The sensitivity problem often leads to complex circuits connected to the sensing elements and/or complex structures associated with the sensing element or the container holding the liquid. Furthermore, liquid level detecting devices which detect a change in capacitance often utilize radio frequency (RF) signals applied or coupled to the sensing element. However, the RF signals can interfere with the operation of other devices coupled to the sensing element and can cause RF emissions unless careful shielding is used.
Thus, there is a need for a liquid level sensing system exhibiting good sensitivity yet which is relatively simple in terms of electronic circuit design and associated structure and which minimizes RF interference and emissions problems.